1. Field of the Invention
The present invention relates to temporary therapeutic devices to work in conjunction with a diseased or failing heart to satisfy the hemodynamic needs of a patient. More particularly, the invention relates to a cardiac assist system for assisting a heart to pump blood, sensing the heart's electrical activity and, when necessary, defibrillating the heart.
2. Discussion of the Related Art
The human heart is a very complicated organ that relies on both mechanical and electrical operation to properly perform. As with any complicated mechanism, problems can and do arise, and the heart is no exception. For example, over time the electrical pathways in the heart (which sequentially cause the atria and ventricles to contract) may fail, thereby causing the heart to lose its rhythm, which is known as arrhythmia. In that event, the ventricles will contract at improper times, and as a result the output of blood decreases.
Diseased hearts may also fibrillate, which results in rapid and irregular contractions of the muscle fibers of the heart. Furthermore, in some failing hearts the heart muscle is no longer able to contract the ventricles to a sufficient extent, also resulting in a dangerous reduction in the amount of blood flow. Thus, the problem in a diseased heart may be mechanical, electrical, or a combination of both.
Numerous attempts have been made to assist these diseased or failing hearts by applying external pressure directly to the heart. One such example is direct manual compression of the heart by a person's hand during open chest cardiopulmonary resuscitation. Often, however, the patient requires cardiac or circulatory support for extended periods of time, such as hours, days, or even weeks, and it is quite difficult for medical personnel to apply a rhythmic pulsating pressure for such an extended period of time. Further, it is difficult if not impossible to apply by hand a uniform compressing force to a significant portion of the exterior ventricle surface of the chamber of the heart. Moreover, the chest should not be opened for extended periods of time because of the increased risk of infection. As such, manual manipulation of the heart is not a solution to the problem in most cases.
To overcome this problem, mechanical devices have been developed to apply external pressure directly to the heart. Some of these devices utilize an inflatable liner that surrounds the heart. For example, U.S. Pat. No. 5,119,804 to Anstadt discloses a cup that is provided with an elastomeric liner. The heart is held in place within the liner, which is cyclically inflated and deflated to apply external pressure to the heart. While this device provides an improvement in hemodynamics for a diseased or failing heart, the device nevertheless suffers from shortcomings, one being the fact that only a fraction of the external fluid pressure that is applied in the cup inlet to displace the liner, which in turn displaces the heart wall, is transmitted to the heart itself to assist the heart to pump blood. As the liner is inflated, because of its axial length limitation, it stretches and bulges radially inwardly. Thus, the transmural pressure of the liner is directed in the radially outward direction (i.e., away from the heart), such that the pressure applied to the heart is less than the pressure applied to the liner. In addition, due to the bulging of the liner, the heart is deformed into a generally hour-glass shape. Thus, there is not a uniform application of pressure to the outer walls of the ventricles. Therefore, a relatively high pressure must be applied within the liner (e.g., 150-200 mm Hg) to support circulation.
Another shortcoming inherent in the prior art devices results from the fact that relatively high pressures are applied almost exclusively to the central portion of the ventricles' outer surfaces. This causes the heart to deform into an unnatural shape and may even eventually cause trauma (e.g., bruises) to the heart, especially if one of those devices is operated for an extended period of time.
In addition, it has been found that these inflatable devices can act as electrical insulators to insulate the heart from externally applied energy, such as the energy from defibrillation paddles. Therefore, when such a device is in place over the heart (which can be on the order of days or even weeks), the heart in many instances cannot be defibrillated. It is believed that the polymer material of the liner device and the air in the liner when it is inflated act as insulators around the heart, preventing the defibrillation current from reaching the heart. Thus, while such devices provide a benefit in the form of mechanically assisting the heart to pump blood, those devices can impede electrical aid from reaching the heart.
Accordingly, it will be apparent that there continues to be a need for a cardiac assist system that automatically applies a compressive force to the ventricles during ventricular contraction to assist the ventricles to pump blood, and that also is operable to electrically stimulate the heart during fibrillation and the like. The present invention addresses these needs.